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Any time that a cat does not eat for more than 24 hours, you should be very concerned. The condition caused by not eating is called hepatic lipidosis where the liver thinks it is being starved and starts "grabbing" fat cells...and actually ends up killing the liver.
I would look at this as the number one cause.
Sneezing, discharge and eye gunk could be an indicator of an upper respiratory infection. These can quickly get serious and kill the cat.
Since you have strays, you might be dealing with FIV/FelV, communicable illnesses that weaken the immune system.
There are so many things here that it "could" be that it would be impossible to list them all.
I am truly sorry for your loss and know that you probably want some closure on this.
My thoughts are with you at this sad time.